So, I uninstalled the Music Manager software, and reinstalled it, and it finally uploaded the new music from the iTunes playlist I added it to. Unfortunately, though, it's now immediately reverting back to having the same issue -- it doesn't "see" new tracks added to the playlist. Which means, every time I want to upload new music, I'm going to have to completely uninstall and reinstall the Music Manager... What a pain...
I don't understand why this issue started occurring in the first place. It's been working fine, catching my new music to upload, for a while now, but all of a sudden, in the last few weeks, it's just not working... :/